In recent years, roller-method sheet supplying devices and air-suction method sheet supplying devices are well known as devices which supply the recording sheets to copying machines or printing machines, serving as the image forming apparatuses.
Concerning the roller-method sheet supplying devices, a sheet supplying roller rotates on the stacked recording sheets to send a single recording sheet one by one, in which sheet separation is conducted by friction force between the roller and the recording sheet, when said friction force is greater than friction force between recording sheets. However, when coated sheets, exhibiting high surface smoothness, are used as the recording sheets in high speed image forming apparatuses, sheet supplying malfunction or double sheets feeding frequently occurs.
Accordingly, the roller method sheet supplying devices have been used for small size printers, due to their low prices. However, said devices are not suitable as the sheet supplying devices which supplies a large number of various type sheets at high speed, whereby the air-suction sheet supplying devices have been generally used for such devices.
The air-suction sheet supplying device includes: a air-suction duct which is arranged to suck air from the recording sheets on a sheet stacking plate; and belts which rotate around the air-suction duct, wherein a plurality of air-suction holes are formed to penetrate the belts, so that air, sucked by the air-suction duct, vacuums the recording sheet through the holes. Since the belts are rotated, the vacuumed recording sheet is supplied to a conveying roller, arranged downstream.
Further, in order to exactly separate a single recording sheet, there is an air-suction sheet supplying device which includes an air blowing section to blow air to a leading edges of the recording sheets, or a sheet surfacing section to blow air at the edges of the recording sheet to separate the recording sheets. Said air-suction sheet supplying device is disclosed on Unexamined Japanese Patent Application Publication 2008-0239312, whereby said device can reliably separate recording sheets one by one, and supply the separated recording sheet downstream.
Concerning the above described air-suction sheet supplying device, if a suction opening of the air-suction duct is determined based on a large size recording sheet, and when small size recording sheets are used, air tends to be blown through the holes which do not cover the small size recording sheet (being air leakage), so that the small size recording sheet cannot be effectively vacuumed. In order to overcome this matter, if air-suction force is to be increased, a high power suction fan is necessary, which results in unwanted sound and increased production cost.
If the suction opening is determined based on a small size recording sheet, when the large size sheets are used, both edges of the recording sheet tend to hang over, so that the recording sheet is not conveyed smoothly, and may be adversely folded.
In order to overcome the above problems, in Unexamined Japanese Patent Publication Application 59-43752, a fan is used so as to change the suction force, based on the size of recording sheets, whereby said fan is controlled to rotate at low speed for the large size sheets, and to rotate at high speed for the small size sheets, in case of the small size recording sheets, air tends to escape from the suction opening.
Further, in Unexamined Japanese Patent Application Publication 06-14467, a shutter is used so as to change the size of the suction aperture, and said shutter is arranged between the suction opening of the air-suction duct and the conveying belt. Based on the size of the recording sheet, the size of the suction aperture can be changed by the shutter.
According to Unexamined Japanese Patent Publication Application 59-43,752, though the fan to change the suction force is provided, when the small size recording sheets are used, air is not prevented from escaping from the suction opening. To overcome air escaping, the fan is rotated at high speed, which is wasteful electrical supply.
According to Unexamined Japanese Patent Application Publication 06-144,617, a sheet, carrying various sizes of opening, serving as a shutter, is driven perpendicular to the recording sheet conveying direction, whereby said shutter sheet is driven by motors, arranged at both sides in the direction to convey the recording sheet, so that the dimensions of suction opening is changed. However, this structure, including the shutter sheet and the driving motors, is very complicated, and results in increased cost of the apparatus.